A power converter is used in various applications, in which a semiconductor switch is switched to control an output voltage and/or an output current. To the power converter is attached a heatsink to radiate heat generated by the semiconductor switch.
FIG. 1 is a schematic side view schematically showing a semiconductor switch equipped on a power converter and an outer appearance of an attached heatsink. FIG. 1 shows a semiconductor switch SW equipped on the power converter. The semiconductor switch SW is attached with a heatsink HS and is fixed on a circuit board B. The heatsink HS has a function of cooling by externally radiating heat generated on the semiconductor switch SW.
However, when the heatsink HS is connected to an FG (frame ground), a parasitic capacitance is generated between the semiconductor switch SW and the heatsink HS.
FIG. 2 is an explanatory diagram schematically depicting the heatsink in a circuit diagram related to the semiconductor switch equipped on the power converter. FIG. 2 shows a circuit in which the semiconductor switch SW is provided to short-circuit between a pair of lines connecting an input side and an output side, and the heatsink HS is shown on the right side of the semiconductor switch SW. The heatsink HS is connected to the FG. Further, a parasitic capacitance Chp is generated between the semiconductor switch SW and the heatsink HS. The generated parasitic capacitance Chp transfers to the FG a voltage fluctuation at a point P1 connected to the positive terminal of the semiconductor switch SW, and whereby a common mode current Icm flows as noise.
FIG. 3 is a graph showing a voltage change at the point P1 of the power converter. FIG. 3 shows a temporal change of a voltage V1 at the point P1, where the horizontal axis represents time and the vertical axis represents the voltage at the point P1. Because the temporal change of the voltage V1 at the point P1 shown in FIG. 3 is transferred to the FG through the parasitic capacitance Chp, the temporal change of the voltage V1 is output as noise to the input side of the power converter.
As described above, when the heatsink HS is connected to the FG, large noise depending on the voltage fluctuation at the point P1 is output to the input side of the power converter. The magnitude of the common mode current Icm to be transferred to the FG and to become noise is represented by the following Equation (A).Icm=Chp×dV/dt  Equation (A)
where
Icm is a common mode current,
Chp is a parasitic capacitance between the semiconductor switch SW and the heatsink HS,
V is a voltage V1 at the point P1, and
t is time.
The parasitic capacitance Chp is represented by the following Equation (B).Chp=ε·S/dhp  Equation (B)
where
ε is a permittivity between the semiconductor switch SW and the heatsink HS,
dhp is a distance between the semiconductor switch SW and the heatsink HS, and
S is an area of the electrode.
In order to reduce the above-mentioned noise, Patent Document 1 proposes a method in which a low-permittivity insulation material using ceramics is provided between the semiconductor switch and the heatsink to reduce the parasitic capacitance generated between the semiconductor switch and the heatsink. Because the parasitic capacitance is reduced, the parasitic capacitance Chp in the above Equation (A) is accordingly reduced, and whereby the common mode current Icm can be smaller.
Further, as another method for reducing the noise, Patent Document 2 proposes a method in which the heatsink itself is connected to a stable potential so that the current causing the noise will be enclosed in a circuit.